Apparatus and method for direct measurement of absorption and scattering coefficients in situ

ABSTRACT

An apparatus for measuring an absorption coefficient includes a first diffusive material, a second diffusive material inside the first diffusive material separated from the first diffusive material by a cavity, and a transparent material proximate to an inner surface of the second diffusive material that holds an absorptive material. First and second light detectors measure light intensities in the first and second diffusive materials  cavity and the transparent material respectively. An absorption coefficient for the absorptive material may be determined based on the first and second light intensities measured when the cavity is illuminated by a light source.

RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. section 119(e) ofthe priority of U.S. Provisional Application No. 60/426,733, filed Nov.15, 2002, entitled “An In Situ Device, System and Method to DirectlyMeasure Both the Absorption and Scattering Coefficients of NaturalWaters.”

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to measuring absorption and scatteringcoefficients of materials, and more particularly to an apparatus andmethod for direct measurement of absorption and scattering coefficientsin situ.

BACKGROUND OF THE INVENTION

It is often useful in analyzing materials to characterize the responseof those materials to incoming light (electromagnetic radiation). Twoknown coefficients for measuring these properties are absorption andscattering coefficients. An absorption coefficient can be expressed asthe property of a medium that describes the amount of absorption ofradiation per unit path length within the medium. It can be interpretedas the inverse of the mean free path that a photon will travel beforebeing absorbed (if the absorption coefficient does not vary along thepath). The unit quantity for an absorption coefficient is inverselength. A scattering coefficient can be expressed as the property of amedium that describes the amount of scattering of radiation per unitpath length for propagation in the medium. It can be interpreted as theinverse of the mean free path that a photon will travel beforeundergoing scattering (if the scattering coefficient does not vary alongthe path). The unit quantity for a scattering coefficient is inverselength. Along with the scattering coefficient, an absorption coefficientdescribes the change in radiation intensity per unit length along thepath through the medium.

Existing approaches to obtaining a scattering coefficient, b, involvemeasuring a volume scattering function using an array of detectorsplaced at various angles around the scattering material. The scatteringcoefficient is then calculated by integrating the volume scatteringfunction over all angles. Traditionally, the scattering coefficient isobtained by measuring the extinction coefficient, c, using the relationb=c−a, where a is the absorption coefficient. The extinctioncoefficient, c, is a constant that predicts the attenuation ordissipation of light at a certain wavelength. In pure water, light ishighly absorbed in the infrared region of the light spectrum and poorlyabsorbed in the blue region. Extinction coefficients are influenced bywater absorption, suspended organic and inorganic particles, anddissolved compounds. Thus, the visible color in a water sample is thelight that is refracted, reflected or re-emitted by substances in waterbecause it has not been absorbed to produce heat or chemical reactions.The absorption coefficient, a, is also measured and, using the relationb=c−a, the scattering coefficient, b, can be calculated. However,sometimes this method can yield unphysical values of the scatteringcoefficient, b.

As for the measurement of the absorption coefficients of light in water,U.S. Pat. No. 5,424,840 to C. C. Moore and J. R. V. Zaneveld describes amethod that can measure the absorption of chlorophyll for a singlewavelength, and which was adapted by C. C. Moore and J. R. V. Zaneveldto be usable for nine discrete wavelengths. The major disadvantages ofthis device are: (1) it cannot measure the scattering coefficientdirectly; and (2) the accuracy of the measured absorption coefficientdepends on how the scattered light is collected and accounted for.Therefore, there remains a need for a device, system and method formeasuring more directly the above coefficients with relative accuracy.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, an apparatusfor measuring an absorption coefficient includes a first diffusivematerial, a second diffusive material inside the first diffusivematerial separated from the first diffusive material by a cavity, and atransparent material proximate to an inner surface of the seconddiffusive material that holds an absorptive material. First and secondlight detectors measure light intensities in the first and seconddiffusive materials respectively. An absorption coefficient for theabsorptive material may be determined based on the first and secondlight intensities measured when the cavity is illuminated by a lightsource.

In accordance with another embodiment of the present invention, anapparatus for measuring a scattering coefficient includes a transparentmaterial that holds an absorptive material, a diffusive material thatsubstantially surrounds the transparent material, and light detectorsthat detect a diffused light intensity in the diffusive material cavityand the transparent material. A light source illuminates the absorptivematerial with a collimated beam. A scattering coefficient for theabsorptive material may be determined based on the incident lightintensity of the collimated beam and the diffused light intensitymeasured by the light detectors when the absorptive material isilluminated.

Important technical advantages of certain embodiments of the presentinvention include direct measurements of absorption and scatteringcoefficients. As noted above, previous techniques often require indirectmeasurements of absorption and scattering coefficients, such as bymeasurement of the extinction coefficient, that can introduce errors inreal-world situations. Other such techniques involve volume integrationof scattered light, which requires mathematical assumptions that may notapply perfectly to real-world situations. In contrast with such methods,certain embodiments of the present invention provide direct measurementof the scattered light intensity.

Other important technical advantages of certain embodiments of thepresent invention include the use of open-ended detectors. Previousdetectors required enclosed containers in order to accurately accountfor all of the light incident on the sample, because of the need forenclosure, introducing samples into the detector proved difficult, oftenrequiring liquid samples to be pumped in and out of the detector. Incontrast with such methods, certain embodiments of the present inventionmay accurately measure absorption and scattering coefficients even whenthe detector is open-ended, thus allowing the sample to be introducedinto the container with relative ease compared to previous methods.

Still other technical advantages of certain embodiments of the presentinvention include durability under real-world use conditions. Previousenclosed detectors often included highly reflective surfaces that wereprone to damage, which resulted in inaccurate measurements. Certainembodiments of the present invention protect optically sensitivecomponents likely to be damaged by exposure to samples, thus providing amore durable detector for in situ use.

Other technical advantages of the present invention will be readilyapparent to one skilled in the art from the following figures,descriptions, and claims. Moreover, while specific advantages have beenenumerated above, various embodiments may include all, some, or none ofthe enumerated advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsadvantages, reference is now made to the following description, taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a rectangular cross-section of a cylindrical detectoraccording to a particular embodiment of the present invention;

FIG. 2 is a perspective view from one end of the detector of FIG. 1;

FIG. 3 is a flow chart illustrating one example of a method formeasuring an absorption coefficient; and

FIG. 4 is a flow chart illustrating one example of a method formeasuring a scattering coefficient.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

FIG. 1 shows a rectangular cross section of a cylindrical detector 100used for detecting absorption and/or scattering coefficients for anabsorptive material 110 within detector 100, and FIG. 2 illustrates aperspective view from the end of detector 100 taken along lines 2—2 ofFIG. 1. In the depicted embodiment, detector 100 includes an outerdiffusive material 102, an inner diffusive material 104, a transparentmaterial 106 between absorptive material 110 and inner diffusivematerial 104, light sources 114 and 116 for introducing light intodetector 100, and detectors 118 and 120 used to measure light intensitylevels at different locations in detector 100. In general, detector 100permits the direct measurement of absorption and scattering coefficientsfor adsorptive material 110.

Outer diffusive material 102 and inner diffusive material 104 comprisematerials that diffuse incoming light such that the light in the mediumof diffusive materials 102 and 104 is approximately isotropic. Thus,scattered photons or other light energy in diffusive materials 102 and104 contributes to the overall intensity of light in diffusive materials102 and 104, but does not produce local variations in light intensity.This allows a measurement of the overall intensity of light in innerdiffusive material 102 cavity 108 and outer diffusive material 104transparent material 106 without regard to positioning sensors in orderto capture particular light rays. In particular embodiments in whichdetector 100 is substantially cylindrical, it is desirable for detector100 to be significantly longer than its diameter in order to minimizeleakage from the ends, thus increasing the accuracy of detector 100. Ina particular embodiment, outer diffusive material 102 and innerdiffusive material 104 may be formed from highly diffusive materialshaving high reflectance, such as those manufactured by SPECTRALON, whichare known in the art as “Spectralon cavities.”

Transparent material 106 forms a tube within detector 100 that separatesabsorptive material 110 from inner diffusive material 104. Transparenttube 106 should be substantially transparent to the wavelength ofinterest being measured for absorptive and scattering properties.Transparent tube 106 is in contact with diffusive material 104 allowinglight to leak from inner diffusive material 104 to absorptive material110 and vice versa. This protects the surface of inner diffusivematerial 104 from contact with absorptive material 110, which mayincrease its durability and protect inner diffusive surface 104 fromdamage resulting from contact with absorptive medium 110. In thedepicted embodiment, transparent material 106 has a wavy inner surfacethat follows a generally sinusoidal curvature. The purpose for thisfeature will be described in greater detail below.

Cavity 108 between outer diffusive material 102 and inner diffusivematerial 104 provides a space for introduction of light into detector100. Because cavity 108 is separated from absorptive material 110 byinner diffusive material 104, light introduced into cavity 108 willreach absorptive material 110 in a nearly isotropic manner. Accordingly,the directional effects for absorption in absorptive material 110 arediminished by the isotropy of the illumination of absorptive material110. Furthermore, the separation between outside diffusive material 102and inside diffusive material 104 created by cavity 108 permitsmeasurement of light intensity both as introduced into detector 100measured in outer diffusive material 102 cavity 108, and afterabsorption by absorptive material 110, measured in inner diffusivematerial 104 transparent material 106. Cavity 108 may in principle befilled with any transparent material including air.

Absorptive material 110 represents any suitable substance to be studiedthat absorbs and/or scatters light. Substances of interest may includewater with suspended particles, plastics, tissues, fluids, or any othersubstance to be characterized by absorptive or scattering properties. Ina particular embodiment, detector 100 may be used to study ocean water110 in real world settings, such as oceanographic vessels.

Light sources 114 represent any suitable source of illumination forcavity 108 in order to allow measure of absorption coefficients. In aparticular embodiment, light sources 114 may be optical fibers coupledto light sources that deliver light from the sources to cavity 108.However, light sources 114 may include wave guides, phosphorescentmaterials, filaments, or other suitable sources of illumination.Depending on the nature of light sources 114, light sources 114 may beisolated from outer diffusive region 102 by a suitable coating or otherbarrier between light sources 114 and diffusive material 102, so thatlight from light source 114 does not increase the light intensity inouter diffusive material 102 other than by leakage from cavity 108. Thisprevents photons from light source 114 that may have directionalproperties from interfering with the isotropy of light in outerdiffusive material 102.

Light source 116 introduces light into absorptive material 110 tomeasure scattering from absorptive material 110. It is desirable forlight emitted from light source 116 to be well collimated so that theenergy from light source 116 is effectively delivered into absorptivematerial 110. This allows the determination of the amount of scatteredlight to be assessed accurately based on the intensity of incoming lightfrom light source 116. The curved inner surface of transparent material106 facilitates scattering measurements by increasing the probabilitythat light scattered at small angles will not be returned intoabsorptive material 110 by specular reflection and travel outside of theend of detector 100. For similar reasons, it is desirable that the lightfrom light source 116 not impinge directly on the surface of transparentmaterial 106.

Light detectors 118 and 120 may include any suitable device formeasuring intensity of light at a desired wavelength. In a particularembodiment, light detectors 118 and 120 are optical fibers that carrylight from outer diffusive material 102 cavity 108 and inner diffusivematerial 104 transparent material 106 to photodetectors that measurelight intensity. Photodetectors may include photodiodes,photomultipliers, photoelectric detectors or any other suitable form oflight detection. In order to increase the accuracy of light intensitydetermination, it may be desirable to isolate light detectors 118 and120 from particular regions of detector 100. For example, light detector118 may be encased in an opaque covering such as aluminum foil in theregion of outer diffusive material 102 and cavity 108 so that themeasurement of light intensity is solely in inner diffusive material 104transparent material 106.

Processor 122 comprises any suitable hardware of software for processinginformation. In particular, processor 122 includes electronic or othertypes of components for receiving information from light detectors 118and 120 and calculating absorption and scattering coefficients based onthat information. Processor 122 may include components for informationstorage (such as magnetic memory), input devices for receivinginformation from detectors and/or users, output devices for displayingor otherwise generating an output of results, and any other appropriatecomponent useful for performing tasks related to the measurement ofabsorption and scattering coefficients.

End caps 112 hold the components of detector 100 in a fixed arrangement.End caps 112 may be ring shaped and may be composed of any suitablematerial that can be affixed to outer diffusive material 102 and innerdiffusive material 104. End caps 112 may also secure transparentmaterial 106 in place as well. End caps 112 may also form part of ahousing (not shown), which may be an integral piece or formed fromseveral pieces. Such a housing may encase the components of detector 100to protect them from exposure to the elements and other hazards andpotentially damaging influences in the environment.

In general, detector 100 should be sized so as to allow adequate opticalseparation between inner diffusive material 102 and outer diffusivematerial 104, typically several millimeters. Furthermore, the length ofdetector 100 along its longitudinal axis (the direction of lines 2—2 inFIG. 1) should be significantly longer than the transverse dimension ofthe sample space holding absorptive material 110 in order to avoid lightleakage. As an example, for a cylindrical, meter-long detector 100, theinner diameter of transparent material 106 could be around tenmillimeters.

In operation, detector 100 may function in one of two modes. In thefirst mode, detector 100 measures the absorption coefficient ofabsorptive material 110. In the second mode of operation, detector 100measures the scattering coefficient of absorptive material 110.

For the first mode of operation, detector 100 is filled with absorptivematerial 110. Light is introduced into cavity 108 by light sources 114.Once equilibrium light state is achieved in detector 100, the lightintensity levels are measured in outer diffusive material 102 cavity 108and inner diffusive material 104 transparent material 106 by lightdetectors 120 and 118, respectively. Based on these measurements theabsorption coefficient of absorptive material 110 may be determined.

In the second mode of operation, light is introduced into detector 100by light source 116. Light is scattered by absorptive material 110,passing through transparent material 106 to diffusive material 104.Diffusive material 104 diffuses scattered light so that the intensity oflight in diffusive material 104 transparent material 106 represents theintensity of light scattered by absorptive material 110. This intensityis measured by light detectors 118, and the scattering coefficient maythen appropriately determined based on the amount of light introducedinto detector by light source 116. Note that the measurement of lightintensity in outer diffusive region 102 cavity 108 is not necessary todetermine the scattering intensity. Accordingly, for a detector 100 thatis used to measure only scattering coefficients, detector 100 may omitouter diffusive region 102 cavity 108, light sources 114, and detectors120. In such a case, inner diffusive material 104 may be encased in areflective material to prevent light from escaping or being absorbed.

The absorption and scattering coefficients may be determined by modelingthe properties of detector 100 based on the particular materials andconstruction used. One example is described for an embodiment in whichdiffusive materials 102 and 104 are Spectralon cavities, transparentmaterial 106 is a quartz tube, and absorptive material 110 is oceanwater. The Spectralon cavities can be modeled as an ideal Lambertianemitter and reflector, which emits equal radiance into all directions.The surface albedo of the Spectralon is taken as 0.994 as an assumption,although other values can be used. It is also reasonably assumed thatthe quartz tube neither absorbs nor scatters the light but does producesome specular reflection. The index of refraction of the quartz isassumed to be 1.46. The water itself has an index of refraction of1.338. A Henyey-Greenstein phase function, given by${{p\left( {\theta,\phi} \right)} = {{\frac{1}{4\quad\pi}{\frac{1 - g^{2}}{\left( {1 - {2\quad g\quad\cos\quad\theta} + g^{2}} \right)^{\frac{3}{2}}} \cdot g}} = \left( {\cos\quad\theta} \right)}},$is used to model the directional dependence of the scattering, as thisallows easy exploration of the effect of the shape of the volumescattering function on the recorded signal. The cylindrical symmetry ofdetector 100 means that the phase function is independent of azimuthalangle φ. The g-parameter is equal to the average cosine of thescattering angle θ. The g=0 case represents isotropic scattering. Theg=0.97 case is chosen to represent a “characteristic upper limit” forreal ocean water, in this case most of the scattered light travelsforward. For an absolute upper limit the value g=0.99 is chosen, in thatthis case can be considered as one in which almost all the scatteredlight travels forward. For most applications with ocean water, thecalculation is not particularly sensitive to variation in g-valueswithin an ordinary range, and accordingly, this variation can beignored, although the calibration curve may include variation forg-values as well if desired. The described relationships can beexploited to determine calibration curves for detectors 118 and 120,which in turn allow subsequent measurements of scattering coefficientsusing detectors 118 and 120.

FIG. 2 illustrates a perspective view from the end of detector 100 takenalong lines 2—2 in FIG. 1. From this perspective, the ring shape of endcap 112 is clearly visible. The inside structure of detector 100 isshown by dashed lines, which illustrate cavity 108 separating outerdiffusive material 102 and inner diffusive material 104. Transparentmaterial 106 extends out from the inner surface of inner diffusivematerial 104 so that transparent material 106 is visible looking throughthe hole at the end of detector 100. Within the space enclosed bytransparent material 106 is absorptive material 110. As depicted on thesides, light sources 114 and detectors 118 and 120 (hidden from thisperspective by detector 118) extend into the sides of detector 100.

Although particular embodiments of detector 100 have been described indetail, it should be understood that these embodiments are onlyexamples, and that numerous modifications and variations of the basicprinciples are possible. For example, as discussed above, detector 100may be enclosed in a variety of housings, and may be assembled in anysuitable manner. If detector 100 does not need to be used in measuringabsorption coefficient, outer diffusive medium 102 and associatedcomponents may be left out of detector 100. In the opposite case, inwhich scattering is not measured, specular reflection from the interfacebetween transparent material 106 and absorptive material 110 is a muchless significant concern, and accordingly, the inner surface oftransparent material 106 may be made smooth. Other physicalconfigurations of detector 100 may also be used in different geometricalsymmetries may be designed, although other geometries may require morecomplicated placements of detectors 118 and 120 and the calculationsperformed based on resulting measurements. These and other variationsshould be understood to be encompassed within the embodiments describedabove.

FIG. 3 is a flow chart 200 illustrating a method for measuring anabsorption coefficient of an absorptive material 110. The first threesteps of the illustrated method are calibration steps to determine theproper settings for detectors 118 and 120, in which detector 100 may befilled with a reference material having known optical properties. Cavity108 is illuminated at step 202. At step 204, light intensity is measuredin inner diffusive material 104transparent material 106and outerdiffusive material 102cavity 108. Detectors 118 and 120 are calibratedaccording to the known absorptive value at step 206. This step may alsoinvolve adjusting the light intensity to determine whether the scalingof the results is appropriate to enable determination of the accuracy ofdetectors 118 and 120.

To measure the absorptive qualities of absorptive material 110,absorptive material 110 is inserted into detector 100 at step 208.Cavity 108 is illuminated using light sources 114 at step 210. Lightintensities are measured in diffusive regions 102 and 104 cavity 108 andtransparent material 106 using detectors 118 and 120 at step 212. Basedon the previous calibration, and the results of the measurement,absorptive coefficient is determined at step 214.

FIG. 4 is a flowchart 220 that illustrates an example of a method formeasuring scattering coefficient. The first three steps of theillustrated method are calibration steps, in which detector 100 may befilled with a reference material with known optical properties. Theinside of the tube formed by transparent material 106 is illuminated atstep 222 while detector 100 is filled with a reference material. Thelight intensity in the inner and outer diffusive materials 102 and104cavity 108 and transparent material 106is measured at step 224.During this step the light intensity produced by light source 116 may beadjusted to ensure that the proper relationship between intensity andscattering coefficient is being observed by detectors 118 and 120. Basedon those measurements, detectors are calibrated at step 226.

To measure scattering coefficient, a light-scattering material isintroduced into detector at step 228. The light-scattering material 110is illuminated using light source 116 at step 230. Light intensity ininner diffusive material 104 transparent material 106 is measured usingdetectors 118 at step 232. The incoming light intensity from lightsource 116 is determined at step 234. This light intensity may be knownfrom the properties of light source 116, or may be measured using photodetectors or other suitable techniques. Based on the measured lightintensity in diffusive material 104 transparent material 106 and thelight intensity incident on light scattering material 110, thescattering coefficient may be determined.

Although the present invention has been described with severalembodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present invention encompass suchchanges, variations, alterations, transformations, and modifications asfall within the scope of the appended claims.

1. An apparatus, comprising: a first diffusive material having agenerally cylindrical shape with a length greater than twice itsdiameter; a second diffusive material inside the first diffusivematerial separated from the first diffusive material by a cavity; atransparent material proximate an inner surface of the second diffusivematerial operable to hold an absorptive material, the transparentmaterial having an inner surface that is curved along its length in asubstantially sinusoidal pattern; first light detectors, comprising:first optical fibers operable to carry light from the first diffusivematerial cavity; and first photodetectors operable to measure a firstlight intensity for the light carried from the first diffusive materialcavity; second light detectors, comprising: second optical fibersoperable to carry light from the second diffusive transparent material;and second photodetectors operable to measure a second light intensityfor the light carried from the second diffusive transparent material;first light sources operable to illuminate the cavity; a second lightsource operable to illuminate the absorptive material with a collimatedbeam having an incident light intensity; and a processor operable to:determine an absorption coefficient for the absorptive material based ona first diffused light intensity measured by the first light detectorsin the first diffusive material cavity and a second diffused lightintensity measured by the second light detectors in the second diffusivetransparent material, wherein the first and second diffused lightintensities are measured while the cavity is illuminated by the firstlight sources; and determine a scattering coefficient for the absorptivematerial based on a third diffused light intensity measured by thesecond light detectors in the second diffusive medium transparentmaterial, wherein the third light intensity is measured while theabsorptive material is illuminated by the collimated beam from thesecond light source.
 2. A method for measuring an absorptioncoefficient, comprising: introducing light into a cavity between a firstdiffusive material and a second diffusive material, wherein at leastsome of the light in the cavity passes into the first and seconddiffusive material and at least some of the light passing into thesecond diffusive material passes through a transparent materialproximate to the second diffusive material and into an absorptivematerial; measuring a first intensity comprising the intensity of thelight in the first diffusive material cavity; measuring a secondintensity comprising the intensity of the light in the second diffusivetransparent material; and determining an absorption coefficient for theabsorptive material based on the first and second intensitymeasurements.
 3. The method of claim 2, wherein the first and seconddiffusive materials are Spectralon cavities.
 4. The method of claim 2,wherein the first and second diffusive materials comprise concentriccylindrical shells; and the cavity comprises a cylindrical space betweenthe first and second diffusive materials.
 5. The method of claim 2,wherein the transparent material comprises a quartz tube.
 6. The methodof claim 2, wherein the transparent material comprises an inner surfacethat is curved along the length of the transparent material.
 7. Themethod of claim 6, wherein the curvation of the inner surface issubstantially sinusoidal.
 8. The method of claim 2, further comprising:stopping the introduction of light into the cavity; after stopping theintroduction of light into the cavity, illuminating the absorptivematerial using a collimated beam, wherein scattered light from theabsorptive material passes through the transparent material to thesecond diffusive material and light is not introduced into the cavitywhile the collimated beam is illuminating the absorptive material; afterilluminating the absorptive material with the collimated beam, measuringa new value of the second light intensity in the second diffusivetransparent material; determining an incident light intensity for thecollimated beam; and determining a scattering coefficient of theabsorptive material based on the incident light intensity and the newvalue of the second light intensity.
 9. An apparatus, comprising: afirst diffusive material; a second diffusive material inside the firstdiffusive material separated from the first diffusive material by acavity; a transparent material proximate to an inner surface of thesecond diffusive material operable to hold an absorptive material; firstlight detectors operable to measure a first light intensity in the firstdiffusive material; and second light detectors operable to measure asecond light intensity in the second diffusive material, wherein anabsorption coefficient for the absorptive material in the transparentmaterial may be is determined based on the first and second lightintensities measured when the cavity is illuminated by a light source.10. The apparatus of claim 9, further comprising a processor operable todetermine the absorption coefficient of the absorptive materials basedon the first and second light intensities.
 11. The apparatus of claim 9,wherein the first and second diffusive materials are Spectraloncavities.
 12. The apparatus of claim 9, wherein: the first and seconddiffusive materials comprise concentric cylindrical shells; and thecavity comprises a cylindrical space between the first and seconddiffusive materials.
 13. The apparatus of claim 9, wherein thetransparent material comprises a quartz tube.
 14. The apparatus of claim9, wherein the transparent material comprises an inner surface that iscurved.
 15. The apparatus of claim 14, wherein the curvation of theinner surface is substantially sinusoidal.
 16. The apparatus of claim 9,further comprising an additional light source operable to illuminate theabsorptive material with a collimated beam having an incident lightintensity, wherein a scattering coefficient may be is determined basedon the second light intensity then the absorptive material isilluminated by the collimated beam and the incident light intensity. 17.A method for measuring a scattering coefficient, comprising:illuminating an absorptive material with a collimated beam having anincident light intensity, wherein the absorptive material is within atransparent material substantially surrounded by a diffusive material;measuring a diffused light intensity in the diffusive transparentmaterial; and determining a scattering coefficient for the absorptivematerial based on diffused light intensity and the incident lightintensity.
 18. The method of claim 17, wherein the diffusive material isa Spectralon cavity.
 19. The method of claim 17, wherein the diffusivematerial comprises a cylindrical shell.
 20. The method of claim 17,wherein the transparent material comprises a quartz tube.
 21. The methodof claim 17, wherein the diffusive material is enclosed by a reflectivematerial having a reflective inner surface.
 22. The method of claim 17,wherein the diffusive material is a first diffusive material, and thefirst diffusive material is substantially surrounded by a seconddiffusive material separated from the first diffusive material by acavity.
 23. The method of claim 22, further comprising: illuminating thecavity; measuring a first diffused light intensity in the firstdiffusive transparent material; measuring a second diffused lightintensity in the second diffusive material cavity; and determining anabsorption coefficient for the absorptive material based on the firstand second diffused light intensities.
 24. An apparatus, comprising: atransparent material operable to hold an absorptive material; adiffusive material substantially surrounding the transparent material;light detectors operable to detect a diffused light intensity in thediffusive material; a light source operable to illuminate an absorptivematerial with a collimated beam having an incident light intensity,wherein a scattering coefficient for the absorptive material may be isdetermined based on the incident light intensity and the diffused lightintensity.
 25. The apparatus of claim 24, further comprising a processoroperable to determine the scattering coefficient for the absorptivematerial based on the incident light intensity and the diffused lightintensity.
 26. The apparatus of claim 24, wherein the diffusive materialis a Spectralon cavity.
 27. The apparatus of claim 24, wherein thediffusive material comprises a cylindrical shell.
 28. The apparatus ofclaim 24, wherein the transparent material comprises a quartz tube. 29.The apparatus of claim 24, wherein the diffusive material is enclosed bya reflective material having a reflective inner surface.
 30. Theapparatus of claim 24, wherein the diffusive material is a firstdiffusive material, and the first diffusive material is substantiallysurrounded by a second diffusive material separated from the firstdiffusive material by a cavity.
 31. The apparatus of claim 30, wherein:the light source is a first light source, the light detectors are firstlight detectors, and the diffused light intensity is a first diffusedlight intensity; the apparatus further comprises: a second light sourceoperable to illuminate the cavity; second light detectors operable tomeasure a second diffused light intensity in the second diffusivematerial; and an absorption coefficient for the absorptive material maybe is determined based on the first and second diffused lightintensities when the cavity is illuminated.